1. Field of the Invention
This invention relates to a motor drive unit for a camera which enables the shutter speed of the camera to be adjustable in accordance with the frame frequency.
2. Description of the Prior Art
In the recently advanced model of single lens reflex cameras, a wide variety of automatic exposure control apparatus of the so-called TTL light metering type have found their use in combination with an electric motor drive unit so that it is enabled to make not only a single frame exposure but also a multiframe exposure or a continuous series of frame exposures, as the cycle of shutter releasing and film winding operation is controlled by the motor drive unit.
Such motor driven cameras may be classified into two main groups, one of which is associated with the shutter priority AE camera, and the other of which is associated with the diaphragm priority AE camera. For the shutter priority AE camera with the associated motor drive, because of the previous setting of an exposure time, a continuous sequence of frame exposure can be made at a constant frame frequency. For the diaphragm priority AE camera with the associated motor drive, because of the occurrence of change in the exposure time each time when an exposure is made as the intensity of light changes, the frame frequency for the continuous sequence can not be maintained constant.
On this account, when the photographer desires to make a certain number of exposures per second in sequence with the conventional motor driven camera, he or she need to manipulate that camera prior to the making of exposures as follows. When the continuous sequence is performed with preselection of a desired frame frequency, as the shutter priority camera is used to permit preselection of a desired shutter speed in accordance with which the diaphragm valve is automatically determined, it is possible to obtain the necessary frame frequency provided that the set value of shutter time is equal to or smaller than the longest possible one corresponding to that frame frequency. However, when this requirement is violated by mistake or by the incapability of the photographer who is a beginner in estimating that longest possible shutter time relative to the desired frame frequency, he or she will fail in achieving performance of the continuous sequence as contradicted to his or her desire. With the diaphragm priority camera, on the other hand, as a desired diaphragm value is previously selected to automatically determine the shutter time as a function of scene brightness, the so-determined value of shutter time is required to be equal to or smaller than the longest possible one dependent upon the selected frame frequency, or otherwise it is impossible to perform a continuous sequence at a constant frame frequency. In consequence, the photographer must know how large shutter time value the exposure meter takes at, and then judge whether or not this value is below the longest possible one. If not so, the diaphragm adjusting means or ring must be operated again. Such repeated settings of the diaphragm ring is troublesome. Further, as the number of frame frequencies available in the motor drive control is increased, the complexity of manipulation of the motor driven camera is increased with increase in the possibility of effecting faulty operation, as the photographer is required to memorize the longest possible shutter time values, or to estimate a necessary one by computation each time he or she has foregotten it.
An object of the present invention is to overcome the above mentioned drawbacks and to provide an electric motor drive unit for a camera in which the set or adjusted value of shutter time in the camera is compared with the longest possible shutter time value dependent upon the selected frame frequency to monitor the shutter time setting or adjustment.
Another object is to provide an electric motor drive unit for a camera in which it is made possible to selectively control the motor drive for photography at a certain constant frame frequency and shutter speed responsive frame frequencies.